If your are experiencing strange behior, e.g. changes to a template are not being displayed on the page, then all caches should be cleared, as follows:
1. Flushing the Drupal cache (click the upper left icon in the Admin Menu for cache flushing options ... very convenient).
2. If that doesn't work, try clearing your browser's cache.
3. As a last resort, try adding a bogus query to the url. If it is being cached by a proxy then this should refresh the source, for example: www.example.com/somepath?refresh=1.
4. Caching problems can sometimes be unbelievable elusive. There are times when you will clear all caches, and for some reason, cached results continue to be displayed. One rule of thumb is to then clear the caches again, then check the results, several times over. If you still suspect that cached results are being displayed, move the Drupal installation to another server, as that will typically enable you to determine if the issues is related to caching.
Tail the Apache error log:
tail /var/log/apache2/error.log (substitute different path if applicable)
You can grep for the error log files if you don't know where they are:
grep 'ErrorLog' /etc/apache2/*
The path to watchdog error logs (D6):
http://www.example.com/admin/reports/dblog
To clear the watch dog error log, use these MySQL commands:
(Change path to MySQL specific to your server)
"C:\Program Files (x86)\acquia-drupal\mysql\bin\mysql" <Database Name> -h 127.0.0.1 -u root -e "DELETE FROM watchdog;"
"C:\Program Files (x86)\acquia-drupal\mysql\bin\mysql" <Database Name> -h 127.0.0.1 -u root -e "ALTER TABLE watchdog AUTO_INCREMENT = 0;"
It is critical that auto increment be reset if you truncate the watchdog table; not doing so may cause errors when viewing the log from within Drupal.
If the getID3 module is installed, make sure the Display Warnings checkbox is not checked, at /admin/settings/getid3.